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Animated Cursor Vulnerability Demystified

With GS, DEP, ASLR, and protected mode IE7, it's possible to go ahead and write a functional proof of concept that will work on Vista and Xp also. When triggering the vulnerability on Vista with a complete overwrite of the return address, the register state looks something like this:

One important point to note is that our malicious ani file contain two ani header one is good i.e of 36 byte and other is the bad boy that cause buffer overflow in LoadAniIcon function.The first is of 36 byte to make sure it passes the loadCursorIconfromFilemap function which checks the header of ani,but the problem is that it is called only once for a single file,if another ani header is encountered then instead of calling loadCursorIconfromFilemap loadAniIcon function will be called.And that gives us chance to hide our malicious code and do the buffer overflow by crafting another ani header in same ani file.

For XP targets, the Metasploit exploit attempts to call through [ebx+4]. The ebx register on Vista has a similar structure, where [ebx] and [ebx+4] point into the RIFF image:

There are also other contextual references that contain parts of, or point into, the RIFF image. The esp and edi registers contain the first ANI header found in the file. The esi register points to an ANIH chunk. For the purpose of this post, only the references found through the ebx register will be considered, though I did investigate these additional avenues to some degree. In Metasploit's existing ANI exploit, you'll see that HD has provided a target for Windows XP that uses a partial overwrite to point the return address at a location that contains a call [ebx+4] instruction. However, there are no immediately equivalent call [ebx+4] instructions within the required 16 page block on Vista:

0:006> s 775b154c L?ef00 ff 53 040:006> s 775b154c L?ef00 ff 63 04

Even though there are no call [ebx+4] instructions, there are a few jmp [ebx] instructions:

This means that if the two low order bytes of the return address are overwritten with 0x700b, the vulnerable function will transfer control into a jmp [ebx] upon return. The jmp [ebx] will start executing code starting with the beginning of the RIFF image itself. The first four bytes of the image is the RIFF chunk tag ("RIFF"). This disassembles to four nop-equivalent instructions shown below:

While these four initial bytes won't cause problems, the four bytes that follow them might. The field that follows the four byte tag is a four byte size which represents the size of the chunk, excluding the header fields. This really isn't much of a problem, though. Since we control the RIFF image that is being generated, we inherently control its size. While we can't reasonbly use all four bytes (since this would require a large RIFF), we can definitely make use of at least the two low order bytes. Padding out the RIFF chunk makes it possible to explicitly control the low order bytes.

With this in mind, the next step is to figure out exactly which instruction the low order bytes of the size field should be set to. We're fairly limited here, but a two byte short jump seems like a good option. Due to the way that the RIFF chunk is set up, its contents will look something like this in memory:

Using this basic layout, we can insert a special embedded chunk as the first entry after the ACON tag. The purpose of this embedded chunk will be to act as a target for the short jump used in the RIFF length field. As such, the embedded chunk should contain additional code to execute. While it's entirely possible to have the embedded chunk contain a payload itself, the Metasploit exploit instead places a long jump that transfers control to the first byte after the end of the RIFF chunk itself. This makes the exploit uniform with XP in terms of how it executes the payload.

I know I left out a lot of details, but let's put the whole thing together. First, you can perform a partial overwrite of the return address using 0x700b. When the vulnerable function returns, it will return into a jmp [ebx] instruction. This will transfer control into the start of the RIFF image, starting with the RIFF tag. The two low order bytes of the RIFF chunk size field can be set to 0x0eb which is a short jump +16. This short jump transfers control into the data portion of an embedded chunk. The data portion of this embedded chunk contains a relative jump of around eb 77 byte to the payload that has been appended after the containing RIFF chunk.

It's definitely possible (and likely) that there are cleaner ways to go about this, but this at least illustrates one way of going about it. It's clear that a partial overwrite of the return address is one of the best solutions in this case. The only major point of contention in this approach centers around what to overwrite the low order two bytes with.

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Steps
1:download the page.
2:open fc4.js in your favourite editor and add following lines in it or just replace it with vode given below.
3:then open the download html file in browser and fill the form with your email and a garbage value string.
4:thats it? it will show you the real security string??
yeah but ...theirs another challenge waiting for you ... :D

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